If you're traveling internationally, make sure that your device is set up to be able to roam. Go to Settings > Network Selection > set to automatic or select supported roaming network provider manually.

If you can't connect to3G/ 4G data connection

Make sure that you have turned on your mobile data on your phone settings

If there is no data connection locally, go to Settings > turn on Mobile Data

If you're traveling internationally, make sure that your device is set up for data roaming. Go to Settings > Mobile Data > Mobile Data Options > set to Roaming On.